Optical devices employing reflectors are used, for example, in displays for laptop computers, hand-held calculators, digital watches and similar devices as well as illuminated signs, light pipes, backlight assemblies and many other devices.
Conventional reflectors, including pigmented surfaces, silvered mirrors, polished metallic or metallized surfaces, etc. suffer from a number of disadvantages in many applications. The conventional reflectors suffer from relatively high absorbance of light incident on their surfaces, typically absorbing about 4–10% of the light incident on them. As a result, the amount of light remaining after each reflection is less than that initially provided. In devices in which multiple reflections are encountered, the overall output of the optical device can be substantially limited. In addition, many of the conventional reflectors are too bulky and/or heavy for many of the applications, particularly in laptop computer displays and other portable devices.
Many optical devices use polarizers, either alone or in combination with reflectors, to provide light having substantially one plane of polarization. Polarized light is especially useful in conjunction with liquid crystal (LC) displays used in many portable devices such as laptop computers and watches, because the LC displays rely on polarized light passing through the LC to display information to a viewer.
Polarizers can be generally categorized as either absorptive or reflective. Typical absorptive polarizers are oriented dyed polymer films, while typical reflective polarizers are tilted thin film polarizers, also known as MacNeille polarizers. Absorptive polarizers do, of course, contribute to the absorptive losses of optical devices in which they are used, thereby limiting the output of those devices.
The absorptive losses of known reflectors and polarizers become much more important when the optical devices are used with a brightness enhancement film such as micro-replicated brightness enhancement film or any other type of reflective polarizer which causes light to typically travel through several reflections, thereby amplifying absorptive losses with every reflection. In the highest gain configurations, for, e.g., a single sheet of brightness enhancement film in combination with a reflective polarizer and back reflector, or two sheets of orthogonally crossed sheets of brightness enhancement film, the effective absorptive losses can reduce the total potential light output of an optical display by 10–30%.
This principle of absorptive losses also applies to optical devices employing non-totally internally reflecting surfaces. One example is an optical wedge in which light is directed into a structure having converging reflective surfaces. Optical wedges will typically reflect light many times before it exits the device. With each reflection, however, some of the light which entered the wedge is absorbed by conventional reflectors. As a result, the amount of light exiting the device is typically substantially less than the light entering the device.
Another optical device typically employing reflective surfaces is an illuminated sign which relies on a finite number of light sources and multiple reflections within an optical cavity to disperse the light to illuminate the surface of a sign in a generally uniform manner. To overcome the problems associated with absorptive losses, many signs typically employ numerous light sources, thereby increasing the cost to manufacture and operate the signs.
Yet another optical device which is limited by absorption losses is a light pipe in which light enters the pipe and is reflected along its length numerous times before exiting at a desired location. Each reflection results in some absorption when conventional reflectors are used, thereby limiting throughput of the light pipe.
To overcome some of the problems of weight, bulk and absorption of conventional reflectors, multi-layered polymer films have been used to reflect and/or polarize light. Such polymeric films are, however, subject to a number of other disadvantages including iridescence, as well as poor reflectivity when off-axis light approaches the surface of the film. The off-axis light is typically transmitted through the films, rather than being reflected, thereby resulting in transmissive losses rather than absorptive losses. Whether light is lost through absorption or transmission, however, the output of the optical device is limited.
Other problems with known multi-layer polymer films used to provide reflectors and/or polarizers is that the materials and methods used to manufacture the films presents serious problems due to poor optical transmission, extrudibility, and high costs.